1. Field of the Invention
The present invention relates to a multi beam scanning type optical writing device, and an image forming apparatus, such as a digital copier, a laser printer, etc, including the multi beam scanning type optical writing device.
2. Discussion of the Background Art
An image bearer included in an image forming apparatus needs a prescribed amount of exposure energy Q per unit area to function. Specifically, a light source needs a light emission output power P calculated by the following formula, wherein alpha represents light usage efficiency of an optical scanning system (i.e., a ratio between emitted light and used light amounts in the system), V represents a process line speed (i.e., a running speed of a recording medium), M represents a number of beams emitted from a light source, E represents a light beam turning on time rate, and L represents a writing width in a main scanning direction;P=(Q·L·V)/(M·E·α).  (Formula 1)
Accordingly, the light emission output power P needs be increased N times when the process line speed V becomes N times (N<1) of the maximum speed Vmax. However, it is demanded that both of the maximum and minimum light emission output powers of a light source device (e.g. a chip) need be suppressed within a prescribed range.
Since a light emission output power of the light source needs be changed to compensate deterioration of the image bearer and adjust developing density or the like, a central value between the maximum and minimum light emission output powers needs be suppressed as minimum as possible as discussed in the Japanese Patent Application No. 2007-293202.
However, when a VCSEL (vertical cavity surface emitting laser) is employed for the light source device, a range between the maximum and the minimum light emission output powers is further narrowed in comparison with a conventional LD (Laser Diode) or a LDA (Laser Diode Array).
To operate in multiple steps of an image formation line speed, the light usage efficiency alpha needs be changed and increases cost.
Further, in an image formation system using a multi beam light emission type VCSEL, reciprocity failure phenomena occurs and causes a banding image at a frequency noticeable to human eyes. Thus, so-called skip scanning described below is needed to employ to suppress the reciprocity failure phenomena.
To appropriately execute the skip scanning without scanning the same line on an image bearer with light beams, intervals between respective neighboring multi beams (i.e., channels) in the sub scanning direction on the image bearer need be partially and symmetrically changed per block of scanning. Thus, a number M of light beams is hardly changed.
The publications of Japanese Patent Application Nos. 2007-293202, 2006-301182, 2004-77714, 2008-209675, and 2007-168299 discuss the related arts.